Switching circuitry

ABSTRACT

Automatic switching circuitry (1) comprises means to process input signals of more than one frequency incorporating information for the derivation of one or more audio channels. The processing means includes selectively variable bandpass means (11, 12) which may be one of several values corresponding to the frequency of an input signal. Means (14) are provided to apply a demultiplex operation on signals output from the bandpass means (11, 12). Control logic (15) monitor the demultiplex means (14) to determine if a demultiplex operation has been achieved and instruct selection of another value for the bandpass means (11, 12) if no demultiplex operation was achieved for a given value of the bandpass means (11, 12).

The present invention relates to automatic switching circuitry,particularly but not solely for use in multi-standard televisionequipment relating to multi-channel audio accompanying conventionaltelevision format signals.

There are many television systems in general use throughout the world.In each of these systems, video signals including colour information aremodulated on a vision carrier and sound signals are modulated on a soundcarrier whose frequency differs from the vision carrier by an amountwhich depends on the television system and on the local standard of thesystem. Also, more systems transmit FM sound signals, although one SECAMsystem transmits AM sound.

With the systems and standards presently in use the spacing between thevision and sound carriers is 4.5 MHz, 5.5 MHz, 6.0 MHz or 6.5 MHzdepending on the television system and standard. Also, spacings of 5.7MHz and 6.2 MHz are coming into use for stereo TV sound. This frequencyspacing is preserved during conversion to the intermediate frequency(IF) in a television receiver, and is converted to the soundintercarrier by balanced demodulation of the sound IF signal by thevision carrier signal filtered from the vision IF signal. Thus, thecarrier frequency of the sound intercarrier signal supplied to the sounddemodulator has one of the values mentioned above.

In conventional television receivers, the sound IF stages and the sounddemodulator contain filters and tuned circuits whose resonantfrequencies are adapted to the system and standard for which thereceiver is designed. Also, the vision IF stages contain resonant trapsfor preventing sound-on-vision interference, and the resonantfrequencies of these traps are similarly adapted to the system andstandard in use.

GB 2124060A (SPT Video) discloses a circuit arrangement to provideautomatic selection of television sound frequency, such as soundintercarrier frequency, in a television receiver for receivingtelevision signals of different systems or standards. The arrangementcomprises a plurality of band pass filters tuned to the different soundintercarrier frequencies. The levels of the outputs of the filters aredetermined by level detecting means and supplied to a circuitarrangement for detecting the highest level of the detected signals.Selection controlling means then selects the frequency of the highestlevel and drives the sound demodulator of the television set.

A proposed transmission system for stereo signals to accompanyconventional television transmissions, known as NICAM, provides a serialdata stream partitioned into 728-bit frames, each transmitted in amillisecond. Each frame has: a first section of eight bits comprising aFrame Alignment Word (FAW) which marks the start of the frame; a secondsection of five bits which provide control information, being one flagbit (namely C_(o), which alternates between 0 and 1 every 8 millisecondsto determine odd and even frames over a 16 frame sequence) and four modebits (namely C₁, C₂, C₃ and C₄, which indicate the nature of thetransmitted signal, e.g. mono, stereo, dual-language, data); a thirdsection of eleven bits of additional data, independent of the controlinformation bits; and finally a fourth section of sixty-four 11-bitwords corresponding to the audio (or data if appropriate) beingtransmitted, this last section having a total of 704 bits.

In each frame as transmitted, interleaving is applied to the block of720 bits which follow the FAW in order to ensure that adjacent bits arenot transmitted sequentially so as to minimise the effect ofmultiple-bit errors. The interleaving pattern places data bits, whichare adjacent in the frame structure as output by the televisionreceiver, in positions at least 16 clock periods apart in thetransmitted bit stream (i.e. at least 15 other bits occur between bitswhich are adjacent in the output frame structure).

In the production of the sound signals, they are sampled at 32 kHz andcoded initially with a resolution of 14 bits per sample. Fortransmission, the number of bits per sample is reduced to 10, usingnear-instantaneous companding, and one parity bit is added to the end ofeach 10-bit sample word for error detection and scale-factor signallingpurposes, thereby resulting in the 11-bit words in the fourth section ofthe frame. At the receiver, the transmitted signals are demultiplexed toproduce an audio output.

An object of the present invention is to provide automatic switchingcircuitry for multi-standard television equipment so that the equipmentis capable of receiving NICAM signals of more than one soundintermediate frequency.

The present invention provides automatic switching circuitry comprisingmeans to process an input signal incorporating information for thederivation of one or more audio channels, the processing meanscomprising:

selectively variable bandpass means,

means to apply a demultiplex operation on signals output from thebandpass means;

means to monitor the demultiplex means to determine if a demultiplexoperation has been achieved;

and means to instruct selection of another value for the bandpass meansif no demultiplex operation was achieved for a given value of thebandpass means.

The present invention allows for transmission of NICAM signals at morethan one sound intermediate frequency (SIF). The inventors haveappreciated that known automatic switching circuitry for multi-standardtelevision equipment is not suitable for selecting between SIFs used forNICAM. In the circuit arrangement disclosed in GB 2124060A, for example,processing of the input signals to determine the SIF to be received bythe sound demodulator takes place before the signals are demodulated. Incontrast, the present invention provides for selection of a value forthe bandpass means, and hence the SIF to be received, after the inputsignal is demultiplexed. Because of the nature of NICAM it is only afterthe input signal at a particular SIF has been demultiplexed that it canbe determined whether or not NICAM signals are being transmitted on thatparticular SIF.

The monitoring means has a first output signal if a demultiplexoperation is achieved and a second output signal if a demultiplexoperation is not achieved and so preferably the monitoring meansincludes means to delay production of said second output signal for atime period so that said second output signal is produced if nodemultiplex operation was achieved over a said time period. This reducesthe risk of the automatic switching circuitry switching away from aparticular value of the bandpass means if there is an instantaneousinterruption in the transmission of a signal.

Preferably the instructing means includes means to latch the bandpassmeans at a particular value if a demultiplex operation is achieved forsaid particular value.

The present invention also embodies equipment with the automaticswitching circuitry as defined herein, and includes equipment for use ina television receiver or a video recording and/or play-back machine,and/or for use in devices associated with the transmission and/orreception of television signals.

In order that the invention may more readily be understood, adescription is now given, by way of example only, reference being madeto the accompanying drawings, in which:

FIG. 1 is a schematic diagram of a part of a television chassisembodying the present invention;

FIG. 2 is a schematic diagram of automatic switching circuitry embodyingthe present invention;

FIG. 3 is a schematic diagram of part of the circuit of FIG. 2;

FIG. 4 is a schematic diagram of an alternative arrangement to thatshown in FIG. 3.

FIG. 1 shows the FM television sound demodulator circuitry which is usedin an existing television receiver chassis and further incorporatesstereo signal processing circuits 1 for processing NICAM signals. Thesound demodulator circuitry is multi-standard and so will process a mainaudio channel carrier of 5.74 MBz to provide stereo sound can also beprocessed. To this effect, the circuitry has three input ceramic filters2, 3, 4 which are in parallel. An FM demodulator 5 double-tuned to 5.5MHz and 6 MHz for the main audio carrier is connected to the first andsecond input ceramic filters 2, 3 which respectively pass signals ofcarrier frequences 5.5 MHz and 6 MHz. The first and second filters 2,3are not switched. A second FM demodulator 6 tuned to 5.74 MHz for thesecond audio channel carrier is connected to the third input ceramicfilter 4 which passes signals of carrier frequency 5.74 MHz.

A SIF signal input to the sound demodulator circuitry is also input tostereo signal processing circuits 1 for processing NICAM signals. Aconventional single standard system for processing NICAM signals has aQPSK decoder, a demultiplexer, a SIF filter and a decoding oscillatorcrystal at a frequency dependent on the single standard for which thecircuit is to be used.

As indicated hereinbefore, NICAM signals comprise digital data. They canbe transmitted by varying the phase of a sinusoidal carrier wave, aprocess termed phase-shift keying (PSK). Quadrature phase-shift keying(QPSK) uses the four phase values 45°, 135°, 225° and 315°. A QPSKdecoder decodes the carrier wave to generate the digital data of whichthe signal is comprised. The data so produced is then demultiplexed toproduce the audio output.

FIG. 2 shows the automatic switching circuitry 1 embodying the presentinvention for processing NICAM signals which operates independent of theFM television sound demodulator operations of FIG. 1. The SIF signal isinput to a set 10 of switchable input bandpass filters, shown in FIG. 2as comprising first and second input filters 11 and respective first andsecond decoding oscillator crystals 12 in switching network. The outputfrom a QPSK decoder 13 is input to a NICAM demultiplexer 14. At presentNICAM signals may be carried on one of two SIF, 5.85 MHz or 6.55 MHz. Inorder for an audio output signal to be produced, the filter 11 anddecoding oscillator crystal 12 corresponding to the SIF of the inputsignal must be selected so that the signal can be passed, decoded and sodemultiplexed.

If the filter 11 and decoding oscillator crystal 12 of incorrect valueare selected, there is no appropriate input signal to the demultiplexer14 and so no demultiplex operation can be achieved. No audio outputsignal is then produced, the demultiplexer 14 being in MUTED mode, i.e.producing a MUTE output signal. Control logic 15 senses the MUTE signaland selects an input filter 11 and decoding oscillator crystal 12 ofanother value. The input filters 11 and decoding oscillator crystals 12are thus switched between various values until that filter and crystalof value corresponding to the SIF of the input signal are selected. Theselection of an appropriate value depends therefore on the QPSK decoder14 actually locking to a valid signal, the NICAM demultiplexer thenUNMUTING, i.e. the processing being done after the demultiplexing of thedata.

The control logic 15 to implement selection of the correct input filterand a crystal utilises a sensing circuit with two stages (see FIG. 3).The first stage monitors the demultiplexer 14 to determine if ademultiplex operation has been achieved and includes a dual rate timeconstant unit 20 and a Schmitt trigger 21. The dual rate time constantunit 20 is driven from the mute output of the demultiplexer 14, therebyallowing rapid charging (unmute) but slow discharge (muting) of the timeconstant reservoir. This level is sensed in the Schmitt trigger circuit21 to give a consistent digital output unaffected by short mutes andinterruptions. The first stage of the control logic 15 accordinglydelays production of a signal indicating that no demultiplex operationwas achieved for a time period dependent on the rate of discharge of theunit 20 and prevents dropout due to instantaneous short mutes.

In the second stage, the control signals for the input filter andoscillator are derived by an astable oscillator 22 running at a veryslow rate (slower than the maximum predicted pull-in time) and enabledin the `mute` mode, i.e. when the first stage of the control logic 15,produces a signal indicating that no demultiplex operation has beenachieved, to switch alternately between the two standards and stoppedwhen the signal is fully locked (represented by an unmuted signal). Alatch 23 holds this value for long enough to confirm this state, untilthe demultiplexer again is muted for longer than the aforementioned timeperiod.

FIG. 4 shows another arrangement of control logic 15 wherein the secondstage is implemented by a microprocessor 30 which is used to sense thestate of the mute line, either polled or by interrupt, and carry out thesearch oscillator and latch function. The microprocessor can also beused to store the default value of standard for any programme thuspotentially reducing the lock-up time of the system at programme change.Storage of the individual channel status can make switchinginstantaneous and automatic.

The function of the dual time constant could also be carried out insoftware in the microprocessor if desired.

We claim:
 1. Automatic switching circuitry for use in receiving NICAMsignals, comprising:filter means for receiving NICAM signals and passingsignals of a certain band; a quadrature phase shift key decoder coupledto receive signals passed by said filter means and for producing adecoded NICAM signal; oscillator means for generation of a plurality ofsignals of different frequencies coupled to said quadrature phase shiftkey decoder; a demultiplexer means receiving the decoded NICAM outputsignals of the quadrature phase shift key decoder for generating a mutesignal responsive to no output signal from the quadrature phase shiftkey decoder and for generating an audio output signal responsivethereto; and control logic means responsive to sensing a mute signalgenerated by said demultiplexer means for switching said filter meansand oscillator means between various values until the quadrature phaseshift key decoder produces a decoded NICAM output signal.
 2. Automaticswitching circuitry according to claim 1 wherein said control logicmeans comprises:monitoring means for monitoring the output of saiddemultiplexer means for determining the existence of a muting signal fora predetermined time; and astable oscillator means for effecting theswitching of said filter means and said oscillator means responsive tothe duration of the muting signal being longer than said predeterminedtime.
 3. Automatic switching circuitry according to claim 2 wherein saidcontrol logic means further includes latching means for latching thefilter means at a particular value if the quadrature phase shift keydecoder produces a decoded NICAM output signal.
 4. Automatic switchingcircuitry according to claim 3 wherein the control logic means formspart of a microprocessor, the microprocessor further comprising means tostore a default value for the bandpass means, the default value beingfor a specific program.